Shades of Gray Hair
A: Once a hair has gone gray, it means that the hair has lost its natural pigment.
For an example of how it works, take a sheet of tissue paper (or tracing paper). Lay the tracing paper over sheets of colored paper as well as another sheet of tracing paper and look at the color of the paper. The more sheets of tracing paper are stacked together, the whiter and more opaque the color becomes, and when laid on top of colored papers, the tracing paper seems to absorb some of the color of the other papers. This is the same with gray hair.
This also explains why when the hair is wet, it appears darker. Even pigmented hair is translucent and reflects and refracts light to some extent. When the hair is wet, the strands cling to one another, becoming less penetrable by the light and appearing darker. This will be doubly true for gray hair, which has less opacity of its own. (Try the tracing/tissue paper test again, but this time mist the tracing paper with water.)
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See also:
Why and how does hair turn gray?
How can I speed up the change of my hair from gray to white?
My hair is snow white, but I detest those old lady white haircuts. Any advice?
My hair is halfway gray from the tips to the middle. Isn't it supposed to be the other way around?